For children (5-10 years) from low-income households, summer represents a ?window of vulnerability? in which weight gain and fitness loss occur at an accelerated rate compared to the school year. We hypothesize the cause of accelerated weight gain and loss of fitness during summer is that children engage in greater amounts of obesogenic behaviors (i.e., diet, sleep, physical activity, sedentary behaviors) during summer that accelerate unhealthy weight gain and fitness loss. Evidence suggests that components within structured days (e.g., school days) have a positive influence on these behaviors. There is also evidence that the routines embedded within structured days are related to children?s self-regulation. During the summer, day camps (e.g., 7AM-5PM, 8-10wks) are a setting that can provide children a structured, healthy environment. However, the vast majority of children from low-income households have limited access to these programs during the summer due to cost. The research gaps addressed in this application are 1) the lack of strategies to mitigate unhealthy weight gain and fitness loss during the summer, 2) and obesogenic behaviors? moderation (or meditation) of children?s weight gain and fitness loss during summer. This study will address these gaps with a rigorous research design that will follow children over two summers and one school year. Children will be randomized to two arms: children who do not attend a structured summer camp, and children who receive a voucher to attend a camp for 8 weeks. We hypothesize that children receiving the voucher will experience less weight gain and fitness loss than children not receiving the voucher. To test this hypothesis the following specific aims will be accomplished: Aim 1 (Primary): Compare changes in BMI z-scores and fitness among children from low-income households attending a structured summer day camp and those not attending a structured summer day camp. Aim 2 (Secondary): Compare changes in obesogenic behaviors among children attending a structured summer day camp and those not attending a structured summer day camp. Aim 3 (Exploratory): Explore changes in self-regulation and the moderating effect of these changes on primary and secondary outcomes among children attending and not attending a structured summer day camp.This study will provide valuable data to inform future interventions tested in a large-scale randomized trial. This project is significant because it targets the most vulnerable population of children (i.e., children from low-income households) during the time when they are the most vulnerable (i.e., summer). This project is innovative because 1) it will provide insight into children?s obesogenic behaviors that are driving unhealthy weight change and fitness loss during the summer and 2) it will provide preliminary data related to the ability of structured days to mitigate unhealthy weight gain and fitness loss during the summer.